Avascular Necrosis of the Hip
Avascular necrosis (AVN) is a generic term that refers to death of bone due to a lack of blood supply. It is also called osteonecrosis. Avascular necrosis of the hip occurs when there is weakening of the bone and subsequent collapse and deformity of the femoral head. If left untreated, arthritis and chondrolysis can occur. AVN in adolescence is frequently referred to as Adolescent AVN and in children is named Perthes Disease.
In both AVN and Perthes Disease, the femoral head loses part (or all) of its blood supply and part (or all) of the bone of the femoral head dies. This dead bone is referred to as necrotic bone. The exact nature of this loss of blood is not well-understood but variety of risk factors have been identified, including:
- Hematologic Diseases (leukemia, lymphoma)
- Dysbaric Disorders (decompression sickness, “the bends”)
- Marrow-replacing Diseases (Gaucher’s Disease)
- Sickle Cell Disease
Avascular necrosis has also been linked to corticosteroid use and alcoholism.
Due to a loss of blood supply to the femoral head, the femoral head gradually weakens and may collapse, leading to deformation of the joint surface. This occurs because the cells that remove dead bone (osteoclasts) remove the “support structure” for the femoral head. If too much bone is removed before the cells can produce new bone (osteoblasts), the femoral head will slowly collapse and the normally sperhically-shaped femoral head will become flattened to an egg-shaped configuration.
There are approximately 20,000 new cases of avascular necrosis reported in the United States each year, and AVN accounts for 10% of total hip arthroplasties performed. It is more common in men than women and bilateral (both hip) involvement is quite common (>80% incidence). When multiple joints are affected, it is termed multifocal osteonecrosis. This, however, is quite rare (<3% incidence).
In the early stages of AVN, the patient will not feel pain and will likely be unaware of the disease progression. Patients will usually present with pain, usually located in the hip, and usually during activities like climbing stairs or inclines, and high-impact activities. Pain will continue to increase as the bone collapses.
Treatment involves preserving the function of the hip joint through reconstructing and preserving the deformed femoral head and ROM treatment to maximize motion and strength of the joint.
Treatment of avascular necrosis depends on the severity and the age of onset. If the disease is caught early, non-operative treatments, such as crutches to redistribute weight on the femoral head, and range-of-motion (ROM) stretching may be recommended. In most cases, however, surgical intervention is necessary.
Early in the disease process, an outpatient procedure called a Core Decompression can be performed. Through a 1 cm incision on the side of the thigh a small drill is used to create a channel into the necrotic bone. This allows a conduit for new blood vessels to enter the femoral head to speed the resorption and repair phase.
Sometimes a synthesized, highly-concentrated bone morphogenic protein (BMP2) is injected into the necrotic area to further aid in healing. A hip abduction brace is worn full time for 6 weeks after surgery and then at night for several months. The surgery becomes more invasive based on the amount of deformity and symptoms, such as stiffness and pain.
We use the technique of hip joint distraction to treat more advanced stages of avascular necrosis. Technically called hip arthrodiastasis, the procedure involves attaching an external fixator to the pelvis and femur. A core decompression can also be performed, and often stiff, contracted muscles are released through small incisions during this surgery. The fixator is used to acutely push the entire femur, including the femoral head, away from the hip joint by approximately 1 cm. This joint distraction unloads the femoral head to prevent further collapse but allows the hip to move into flexion and extension to prevent stiffness. The fixator is removed after 14 weeks and a brace is used as with the core decompression.
Advanced stages of avascular necrosis can be treated with a more involved surgery called a Safe Surgical Dislocation and Femoral Head Reduction Osteotomy (FHRO). Originally described in Sweden by Dr. Ganz in 2001, this is a complex hip surgery performed routinely at only a few specialized centers in the world. We have modified the original procedure and have been performing it for AVN and Perthes for over a decade with excellent long-term results.
We start with a surgical dislocation of the hip, where the femoral head is removed from the hip joint and visualized. The necrotic bone is then excised and a bone graft applied. We also inserts bone-morphogenic protein (BMP), a growth factor that induces bone to grow. A membrane is then inserted over the bone graft and the BMP, reshaping the femoral head back into a spherical shape. We will then apply an external fixator and distract the hip out of the joint. The external fixator unloads the femoral head, giving it time to heal. The body’s weight is supported through the fixator which crosses the hip joint.
Once the femoral head has healed, the external fixator is removed and the femoral head is supported through internal screws. Physical therapy is necessary in order to regain and maximize hip range-of-motion (ROM) and strength.